The present invention relates to a four-wheel drive system for an automotive vehicle having at least a front and a rear pair of road wheels. More particularly, the present invention relates to improvements in a transaxle mechanism for use in a four-wheel drive system of an automotive vehicle.
A transaxle mechanism is disclosed in European Pat. No. 0043 806. This patent discloses a motor vehicle having a transmission system which includes a toothed wheel, an epicyclic gear train, a spider, front shafts, a bevel gear, a bevel pinion and a rear differential. The toothed wheel is driven by a gear carried by a secondary shaft of a gear box and then the toothed wheel drives the epicyclic gear train. The spider is driven by the epicyclic gear train and then the spider drives the front shafts.
The bevel gear is fixed to a planet carrier of the epicyclic gear train and the bevel pinion is meshed with the bevel gear. When the planet carrier is driven by the toothed wheel, the bevel pinion is driven by the bevel gear and then the rear differential is driven.
The transmission system can transmit driving torque to the front wheels through the toothed wheel, the epicyclic gear train, the spider and the front shafts, and can transmit driving torque to rear wheels through the toothed wheel, the epicyclic gear train, the bevel gear, the bevel pinion, the rear differential and rear shafts.
As shown in FIG. 1 of European Pat. No. 0043 806, a propeller shaft of the transmission system extends from an adjacent portion of an end of an engine. When the transmission system is mounted on the vehicle body, the propeller shaft may be located at one side of the vehicle body to balance the weight of the vehicle. This often results in the propeller shaft being located under passengers' seats, thus, creating problems in the arrangement of the passenger compartment. Further, as shown in FIG. 2 of European Pat. No. 0043 806, the toothed wheel is supported on a casing by a first roller bearing and the planet carrier is supported on the casing by a second roller bearing. However, there is no support means between the toothed wheel and the planet carrier. Accordingly, the toothed wheel and the planet carrier are not securely supported in the longitudinal direction of the front shafts and as a result, the toothed wheel is not securely engaged to the epicyclic gear train.
A second transaxle mechanism is disclosed in U.S. Pat. No. 4,476,953. This patent discloses a four wheel vehicle drive system including a transmission output gear, a central differential gear assembly, a first intermediate gear which is fixed to a side bevel gear of the central differential gear assembly, an intermediate gear shaft, a bevel gear which is fixed to the intermediate gear shaft, a driven bevel gear which is connected to a propeller shaft, a rear differential gear assembly, a front-wheel differential gear assembly and front shafts.
The central differential gear assembly is driven by the transmission output gear and the side bevel gear of the central differential gear assembly drives the first intermediate gear. The intermediate gear shaft is driven by the first intermediate gear and the bevel gear drives the driven bevel gear. The rear differential gear assembly is driven by the propeller shaft. Further, the side bevel gear of the central differential gear assembly drives the front-wheel differential gear assembly which drives the front shafts.
Therefore, the four-wheel vehicle drive system can transmit driving torque to front wheels through the transmission output gear, the central differential gear assembly and the front shafts, while the four-wheel vehicle drive system can transmit driving torque to rear wheels through the transmission output gear, the central differential gear assembly, the first intermediate gear, the intermediate gear shaft, the bevel gear, the driven bevel gear, the propeller shaft, the rear differential gear assembly and rear shafts.
Further, in the four-wheel vehicle drive system, the first intermediate gear is located apart from the central differential gear assembly and is located adjacent to the center portion of the vehicle in the lateral direction of the vehicle.
However, the front differential gear assembly of U.S. Pat. No. 4,476,953 is located apart from the central differential gear assembly, and, as a result, a hollow shaft is required. Accordingly, the weight and size of the four-wheel vehicle drive system is increased.